As an improvement over the straight slot on conventional screw heads, the Torx™ slot was developed. This had the general shape of a star and extended downwardly into the screw head, giving better engagement characteristics than the straight slot. Other screws had shaped recesses in their screw heads and had straight or near straight sidewalls. Examples are Phillips, Hex or Spline headed screws. A tool (screwdriver) of complementary shape was used to drive the screw into whatever backing material was appropriate. If the shaped slot extended too far downwardly into the head of the screw, the lowest parts of the extremities of the slot could approach the undersurface of the screw head. For instance the shaped slot might approach the external conical undersurface of a countersunk screw, so as to weaken the screw head. Several systems have been developed to overcome this and other disadvantages.
Examples of two such systems are shown in two earlier patents with which the present applicant was associated. These are U.S. Pat. Nos. 6,951,158 and 7,730,812. U.S. Pat. No. 6,951,158 gives a detailed review of the prior art which will not be repeated.
In specification U.S. Pat. No. 7,730,812 there is a screw head having a hexalobular slot in its upper surface. The curved wall of the slot is approximately parallel to the axis of the screw throughout its length, and may be tapered downwardly inward at an angle which may typically be 1.5 deg. At the base of the hexalobular slot there is a downwardly and inwardly tapered transition surface, leading to further a central recess of smaller diameter. This smaller further recess has tapered walls. Practical experience has led to the use of more broadly tapered walls, which may be tapered downwardly at between 6 deg and 12 deg, and for instance at 10.8333 deg to the axis of the screw, typically between 6.5 deg and 7.5 deg and preferably at 7.5 deg.
The intention of the further recess was to promote the accurate insertion of an appropriately shaped tool (screwdriver) within the hexalobular slot of the screw head. Engagement of a central end point of tapered circular cross section on the tip of the tool into the further recess allowed the screw to be retained on the tool while a craftsman manoeuvred the screw into position.
Retention of the screw on the tool by press engagement is particularly useful on one-handed power tools and for robotic tools.
Other screw and tool systems are known to have been manufactured and marketed. Examples are the ‘anti cam out’ ACR™ system and the Ribbed Phillips™ ‘anti-cam out’ rib system. These systems have a drive system with ribs, but in these examples the ribs extend down into the screw heads in one unbroken length, and have no transition slopes within the screw head recesses.
The present invention is intended to make a significant improvement inter alia on the system disclosed in U.S. Pat. No. 7,730,812.